The present invention relates to a method of removing volatile organic material from an aqueous dispersion of synthetic polymer. In particular it is directed to a method for economically removing residual organic components following the polymerization of organic monomers in an aqueous medium to form a dispersion of polymer in water. At the end of the polymerization stage in the manufacture of polymer dispersions, the dispersion typically contains residual organic impurities which result from incomplete conversion of monomers, impurities in raw materials and undesirable by-products formed during the polymerization reaction. For example, in a styrene-butadiene aqueous emulsion polymer, these impurities would include: unreacted styrene; ethyl benzene, an impurity in the styrene; 4-vinyl cyclohexene, an impurity in butadiene; 4-phenyl cyclohexene, a by-product of the polymerization.
Conventionally, organic volatiles have been removed from an aqueous polymer dispersion made by emulsion or suspension polymerization by a process known as steam or inert gas stripping, by contacting the dispersion with steam or an inert gas at either reduced or elevated temperatures and pressures, this process being carried out in a variety of different types of equipment, for example, columns, semibatch strippers, thin film evaporators and plate evaporators. However, because of the strong interaction between the organic volatiles and the polymer, to achieve a low level of volatiles in the dispersion requires high ratios of stripping medium/dispersion. To generate this stripping medium requires the use of large amounts of energy and results in a high energy cost. The higher the boiling point of the organic material being removed and the lower the required residual level in the dispersion, then the higher is the energy cost.
Another practiced technology in the industry is to use a chemical or combination of chemicals which produces free radicals to cause further polymerization of monomers. However, this process does not reduce the level of non-monomeric species, such as the Diels-Alder condensation products of butadiene or impurities such as ethyl benzene.
A combination of steam/inert gas stripping coincident with further polymerization of monomers is also practiced.
Many of the processes in use in the industry involve prolonged exposure of dispersion to conditions of elevated temperature and/or high mechanical shear in equipment designed to improve contact between the dispersion and the stripping medium. These processes do not remove high boiling components. Accordingly, it would be desirable to have a process for removing high boiling point organic volatiles from an aqueous polymer dispersion down to very low levels, which does not involve the prolonged exposure of the dispersion to high temperature or mechanical shear and which is overall an improved energy efficient process.